Coherent optical receiver

ABSTRACT

An optical IQ demodulator that does not require a power-consuming DSP is disclosed. A DC offset is added to one of the I and Q optical signal components at an IQ transmitter. After mixing with an LO signal and differential detection at the receiver, this DC offset results in a heterodyne-frequency tone in each of the quadrature detection channels of the receiver. The phase of this oscillation is recovered using a PLL circuit, which output is used to separate the transmitter I and Q channels for decoding thereof using conventional electronics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Patent Application No.16/223,853, filed Dec. 18, 2018, now allowed, which is a continuation ofU.S. patent application Ser. No. 15/841,789, filed Dec. 14, 2017, nowU.S. Pat. No. 10,205,535, which are hereby incorporated by referenceherein in their entireties.

FIELD OF THE INVENTION

The invention generally relates to coherent optical receivers, and moreparticularly relates to an apparatus and method for demodulating opticalquadrature modulated signals.

BACKGROUND OF THE INVENTION

Optical waveguide modulators used in high-speed optical communications,such as those based on waveguide Mach-Zehnder (MZ) interferometricstructures, may require active control of their operating conditions,and in particular of their bias voltage that sets the relative phase ofinterfering light waves in the modulator in the absence of themodulation signal. The waveguides of the modulator are typically formedin an electro-optic material, for example a suitable semiconductor orLiNbO₃, where optical properties of the waveguide may be controlled byapplying a voltage. Such a waveguide modulator may be a part of anoptical integrated circuit (PIC) implemented in an opto-electronic chip.

Very high speed optical systems may benefit from one of quadraturemodulation (QM) formats such as the Quadrature phase shift keying (QPSK)and Quadrature Amplitude Modulation (QAM). These modulation formats maybe realized using a quadrature modulator which is typically implementedusing nested MZ interferometric structures. For example, a QAM opticalsignal may be generated by splitting light from a suitable light sourcebetween two MZ modulators (MZM) that are synchronously driven by anin-phase (I) modulation signal and a quadrature (Q) modulation signalthat carry respective I and Q components of an electrical QAM or QPSKsignal, and then combining the resulting I-channel and Q-channel lightsignals in quadrature, i.e. with a relative optical phase shift ϕ_(IQ)equal to 90°, or π/2 radians (rad). For example the two MZMs of aquadrature modulator may each be modulated by a BPSK (binary phase shiftkeying) signal while being biased at their respective null transmissionpoints for push-pull modulation. When their outputs are added togetherin quadrature, i.e. with the relative phase shift ϕ_(IQ)=π/2, a QPSKsignal (Quaternary phase shift keying) results.

At a receiver site, the QM modulated signal may be coherently combinedwith light from a local oscillator (LO) source, typically using a 90°optical hybrid, which outputs are coupled to one or more differentialreceivers. The phase of the LO light relative to the received lightsignal is however typically unknown, and digital signal processing isconventionally used to perform phase recovery and extract thetransmitter I-channel and Q-channel signals from the detected electricalsignals at the outputs of the differential detectors. This signalprocessing may be relatively complex and thus typically requirerelatively power-consuming and expansive digital signal processors.

Accordingly, it may be understood that there may be significant problemsand shortcomings associated with current solutions and technologies fordemodulating optical quadrature modulated signals.

SUMMARY OF THE INVENTION

An aspect of the present disclosure relates to a method for coherentdemodulation of quadrature-modulated (QM) light comprising an I-channelsignal and a Q-channel signal, the method comprising: a) at atransmitter site, providing an offset in one of the I-channel andQ-channel signals; b) at a receiver site, performing optical quadratureheterodyne detection of the QM light to obtain two electrical signals,each of the electrical signals comprising the I-channel signal modulatedat a heterodyne frequency (HF), the Q-channel signal modulated at the HF(UHF, and a heterodyne frequency (HF) tone; c) filtering the twoelectrical signals at the HF (UHF with a narrow-band phase-sensitivefilter to extract the HF tones therefrom; and, d) using the HF tonesobtained in c) to decompose the I-channel and Q-channel signals from thetwo electrical signals.

Performing the optical quadrature heterodyne detection may comprisemixing the QM light with local oscillator (LO) light using a 90 degreesoptical hybrid (OH), wherein the LO light is shifted in frequency fromthe QM light by the HF ω_(HF); and, detecting mixed optical signals fromoutputs of the OH using two differential photodetectors to obtain thetwo detected electrical signals.

In one implementation of the method, providing the offset in one of theI-channel and Q-channel signals may include providing a DC bias offsetto an optical modulator driven by one of the I and Q channel signals.

An aspect of the disclosure provides an apparatus for coherentdemodulation of quadrature-modulated (QM) light comprising an I-channelsignal and a Q-channel signal, wherein the QM light comprises an offsetin one of the I-channel and Q-channel signals. The apparatus maycomprise a first optical receiver that comprises an optical heterodynereceiver and an IQ demodulator. The optical heterodyne receiver isconfigured to perform optical quadrature heterodyne detection of the QMlight to obtain two electrical signals, each of the electrical signalscomprising the I-channel signal modulated at a heterodyne frequency(HF), the Q-channel signal modulated at the HF (UHF, and a heterodynefrequency (HF) tone. The IQ demodulator comprises a phase-sensitivenarrow-band filter circuit configured to extract the HF tones from thetwo electrical signals, and an HF demodulation circuit, and an HFdemodulation circuit coupled to the phase-sensitive narrow-band filtercircuit and the optical heterodyne receiver and configured to decomposeeach of the I-channel and Q-channel signals from the two electricalsignals using the HF tones extracted therefrom. The HF demodulationcircuit may comprise a network of four signal multiplication circuitsand two signal summing circuits.

In accordance with an aspect of the present disclosure, the apparatusmay be configured to receive polarization multiplexed (PM) light whereinthe QM light is polarization multiplexed with a second QM light, andwherein the QM light comprises a distinct dither signal that is absentfrom the second QM light. The apparatus may include a tunablepolarization splitter disposed to receive the PM light and configured totunably split the PM light into two light signals responsive to acontrol signal, the tunable polarization splitter comprising a firstoutput port optically coupled to the first optical receiver and a secondoutput port. A second IQ demodulator may be optically coupled to thesecond output port of the polarization splitter. A dither detectioncircuit may be coupled to one of the two output ports of the tunablepolarization splitter and may be configured to detect the distinctdither signal therein and to output a dither strength signal indicativeof strength of the detected distinct dither signal. A control circuitmay be coupled to the tunable polarization splitter and configured tovary the control signal so as to minimize or maximize the strength ofthe first dither signal thereby aligning the two light signals totransmitter polarization channels.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments disclosed herein will be described in greater detail withreference to the accompanying drawings, which may be not to scale and inwhich like elements are indicated with like reference numerals, andwherein:

FIG. 1 is a schematic diagram of an optical communication link usingquadrature modulated optical signals and coherent reception;

FIG. 2A is a graph illustrating a constellation of a conventional 16-QAMoptical signal;

FIG. 2B is a graph illustrating a constellation of a 16-QAM opticalsignal with a DC shift in the “I” modulation signal component;

FIG. 3 is a schematic diagram of a coherent optical receiver circuitincluding a PLL-based IQ demodulation circuit;

FIG. 4 is a schematic diagram of an embodiment of a quadrature signalrotator used in an IQ demodulation circuit of FIG. 3;

FIG. 5A is a schematic diagram of an optical IQ modulator in the form ofa nested Mach-Zehnder interferometer for generating opticalquadrature-modulated signals;

FIG. 5B is a schematic diagram illustrating offset biasing of aMach-Zehnder modulator relative to an EO transfer curve thereof;

FIG. 6 is a is a schematic diagram of a dual-polarization (DP) opticalIQ modulator with two nested Mach-Zehnder interferometers for separatelymodulating light of X and Y polarizations, and a dither modulator in oneof the X and Y polarization branches;

FIG. 7 is a schematic block diagram illustrating an optical receivercircuit for demodulating signals generated using the DP modulator ofFIG. 4 with optical-domain polarization recovery and analog IQdemodulation;

FIG. 8 is a flowchart of a method for demodulation of an optical QMsignal with a transmitter DC offset.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular opticalcircuits, circuit components, techniques, etc. in order to provide athorough understanding of the present invention. However, it will beapparent to one skilled in the art that the present invention may bepracticed in other embodiments that depart from these specific details.In other instances, detailed descriptions of well-known methods,devices, and circuits are omitted so as not to obscure the descriptionof the present invention. All statements herein reciting principles,aspects, and embodiments of the invention, as well as specific examplesthereof, are intended to encompass both structural and functionalequivalents thereof. Additionally, it is intended that such equivalentsinclude both currently known equivalents as well as equivalentsdeveloped in the future, i.e., any elements developed that perform thesame function, regardless of structure.

Furthermore, the following abbreviations and acronyms may be used in thepresent document:

GaAs Gallium Arsenide

InP Indium Phosphide

PIC Photonic Integrated Circuits

SOI Silicon on Insulator

PSK Phase Shift Keying

BPSK Binary Phase Shift Keying

QAM Quadrature Amplitude Modulation

QPSK Quaternary Phase Shift Keying

DC Direct Current

AC Alternate Current

DSP Digital Signal Processor

FPGA Field Programmable Gate Array

ASIC Application Specific Integrated Circuit

In the following description, the term “light” refers to electromagneticradiation with frequencies in the visible and non-visible portions ofthe electromagnetic spectrum. The term “optical” relates toelectromagnetic radiation in the visible and non-visible portions of theelectromagnetic spectrum. The terms “first”, “second” and so forth arenot intended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. The word‘using’, when used in a description of a method or process performed byan optical device such as a polarizer or a waveguide, is to beunderstood as referring to an action performed by the optical deviceitself or by a component thereof rather than by an external agent.Notation Vπ refers to a bias voltage of a Mach-Zehnder modulator (MZM)that corresponds to a change in a relative phase delay between arms ofthe MZM by π rad, or 180 degrees, which corresponds to a change from aminimum to a next maximum in the MZM transmission.

The term “90° optical hybrid” refers to an optical device that combinestwo input optical signals ‘S’ and ‘LO’ to produce four mixed outputoptical signals in which the two input optical signals are added with anoptical phase shift ϕ₁₂ that increments by 90°, or π/2 radian, from oneoutput to another.

One aspect of the present disclosure relates to receiving anddemodulating optical quadrature-modulated signals. Such signals aretypically generated at a transmitter site by combining two modulatedoptical signals in quadrature, i.e. with an optical phase shifttherebetween of 90 degrees, or π/2 radian. The two optical signals beingadded at the transmitter in quadrature are commonly referred to as the I(in-phase) optical signal and the Q (quadrature) optical signal. It willbe understood however that this designation is somewhat arbitrary andcan be reversed by selecting a different initial phase. The I and Qoptical signals are each independently PSK and/or ASK modulated with acorresponding electrical I or Q modulation signal, so that the resultingtransmitter light is QAM or QPSK modulated.

FIG. 1 generally illustrates an example optical transmission system 5using QM optical signals. At a transmitter site 10, an opticaltransmitter (Tx) 15 receives two electrical data signals of theI-channel and the Q-channel, denoted un the figure as I(t) 12 and Q(t)13 respectively, and uses them as modulation signals to modulate twolight beams, which are then combined in quadrature with the 90 degreesoptical phase shift therebetween to generate a QM optical signal 11S(t). Mathematically this signal may be described by the followingequation (1):

S(t)=S ₀·ρ(t)·exp(jω _(t) t+ϕ(t)) )   (1)

where ρ(t) and ϕ(t) are the modulated amplitude and phase of the QMoptical signal 11, ω_(t) is the frequency of the optical carrier, and iis the imaginary unit.

FIG. 2A illustrates by way of example a constellation corresponding to a16-QAM optical signal that may be generated with the Tx 15, showing allpossible values of a 16-QAM symbol in a complex (I, Q) plane. Each point210, shown with an open circle, represents one possible combination (ϕ,ρ) of the optical phase ϕ and the optical amplitude p of a 16-QAM symbolin the (I, Q) plane of the transmitter. For the 16-QAM example, each ofthe I and Q components can have 4 possible values. Hence, a total of2×2×2=8 bits can be sent in total for each QAM symbol period. Using the16QAM modulation, a 50 Gbaud signal can carry 400 Gbit/s of information.

Referring back to FIG. 1, the Tx-generated QM optical signal 11propagates along a fiber-optic link 21 to a receiver site 30, where itis to be demodulated, i.e. the transmitter modulation signals I(t) andQ(t), which carry data of the respective information channels,separately extracted in the form of two electrical data signals. Thismay be done using a coherent optical receiver (Rx) 31 in which thereceived optical signal 11 is coherently mixed with reference light, forexample generated by a local oscillator (LO) source 33, and withphase-shifted versions of the reference light, to produce another pairof electrical signals I_(r)(t) and Q_(r)(t) that are in a quadraturerelationship to each other. This optical mixing and EO conversion may beperformed, for example, using a 90 optical hybrid (OH) coupled to a pairof differential photodetectors. However, since the relationship betweenthe optical phase of the LO light 32 and the received light 11 istypically unknown, each of the Ir and Qr signals extracted at thereceiver includes both the transmitter I-channel signal and thetransmitter Q-channel signal. As the optical phase of the LO light istypically not locked to that of the QM light 11 generated at thetransmitter, a phase recovery mechanism is required at the receiver site30 to obtain the transmitter I(t) and Q(t) signals from the receiverdetected quadrature signals Ir(t) and Qr(t). The phase recovery and theseparation of the I and Q transmitter channels is a non-trivial task andis typically performed by a suitably programmed digital signal processor(DSP) 35.

Turning now to FIG. 2B, there is illustrated a constellation diagram ofa 16QAM signal that is slightly modified in accordance with an aspect ofthe present disclosure, by adding a DC offset 222 to one of the twoquadrature components thereof, in the shown example—to the I-channelmodulation signal. The constellation of the modified 16QAM signal withthe DC offset is shown with crosses 211, while the constellation of aconventional 16QAM signal is shown in the same figure with open circles210. Advantageously, the addition of this DC offset 222 enablessimplifying the phase recovery at the receiver and the separation of theTx-generated I and Q modulation signals, potentially eliminating theneed for a receiver DSP.

With reference to FIG. 3, there is schematically illustrated a blockdiagram of an example embodiment of an optical coherent receiver that isconfigured to make use of the presence of a DC offset 222 in thereceived QM signal for phase recovery and IQ demodulation. The receiveruses a simplified IQ demodulator circuit 350 to separate theTx-generated I and Q signals of a received QM optical signal 301 S(t)having the DC offset 222 in one of the quadrature modulation componentsthereof. Similarly to a conventional coherent optical QM receiver, thereceiver of FIG. 3 includes at its front-end an electro-optical (EO)heterodyne converter 340, which may also be referred to as theheterodyne detector 340, and which includes a 90° OH 310 followed by apair of differential photodetectors 320. The OH 310 has two input ports,one for receiving the QM light 301 and another for receiving LO light302 from an LO source 305. The LO source 305 may generate light thatslightly differs in frequency from the QM signal light 301 to providefor a heterodyne detection. The difference in frequency ω_(HF) betweenthe QM light 301 and the LO light 302, which may be referred to hereinas the heterodyne (HF) frequency, may be, for example, in the range offew MHz to few hundred MHz, but may also be outside of this range. Thereceived QM optical signal 301 S(t) and the LO optical signal 302 S_(LO)at the OH inputs may be represented by the following two equations (2)and (3), respectively:

The QM optical signal S(t) having this DC offset 222 may be described bya following equation (2):

S(t)=(A ₀ +A(t))·sin(ω_(t) t+ϕ _(t))+B(t)·cos (ω_(t) t+ϕ _(t)),   (2)

S _(LO) =C sin(ω_(l) t+ϕ _(l))   (3)

where A(t) and B(t) represent the Tx-generated I and Q modulationsignals, respectively, ω_(t) is the optical frequency of the QM light,ω_(l) is the optical frequency of the LO light, ϕ_(t) and ϕ_(l) are theoptical phases of the QM and LO lights at the point of mixing in the OH310.

In operation OH 310 outputs four mixed optical signals that are denotedas I+, I−, Q+, and Q−, and in which the signal light S 301 and the LOlight 302 are coherently mixed with an optical phase shift therebetweenthat increments by 90° from one output signal to another. Four outputoptical ports of the OH 310 are coupled to the two differentialphotodetectors (DPDs) 320 in a manner known in the art, so that eachdifferential detector receives two mixed signals in which the signallight S 301 and the LO light 302 are mixed with a 180 degrees opticalphase shift therebetween. Differential PDs 320 generate two electricalsignals V₁(t) 341 and V₂(t) 342 that may satisfy the following equations(4) and (5):

V ₁ =I ₊ −I ⁻ =C·[(A ₀ +A(t))·CC(t)−B(t)−SS(t)]  (4)

V ₂ =Q ₊ −Q ⁻ =C·[(A ₀ +A(t))·SS(t)+B(t)·CC(t)]  (5)

where C is a constant and CC(t) and SS(t) are two quadrature harmonicsignals or tones at the heterodyne frequency (HF) ω_(HF)=(ω_(t)−ω_(l)):

CC(t)=cos((ω_(t)−ω_(l))t+((ϕ_(t)−ϕ_(l)))   (6)

SS(t)=sin((ω_(t)−ω_(l))t+(ϕ_(t)−ϕ_(l)))   (7)

Thus each of the two electrical signals 341, 342 is proportional to amixture of the Tx-generated I and Q signals that are modulated with thequadrature HF tones CC(t) and SS(t) given by equations (6) and (7). Inthe absence of the DC offset 222 in the received QM signal 301, i.e.when A₀=0, the I and Q signals are not easy to separate from theelectrical signals 341, 342 since the LO-signal phase difference(ϕ_(t)−ϕ_(l)) is generally not a priory known and may fluctuate in time.Thus, a phase recovery operation may generally be required, and isconventionally performed using a DSP. However, in the presence of the DCoffset A₀ the phase recovery is simplified as the two quadrature HFtones CC(t) and SS(t) may be relatively easily extracted from theoutputs 341, 342 of the EO converter 350, as they are separately presentas additive HF tones in the electrical signals 341 and 342; thus theseHF tones may be extracted from the outputs 341, 342 of the heterodynedetector 340 using narrow-band filter circuits that preserve theirrelative phase, such as for example a two-channel PLL (phase lock loop).

With the modulating HF tones CC(t) and SS(t) known, the Tx generated Iand Q signals A(t) and B(t) can be decomposed, i.e. separatelyextracted, from the output signals 341, 342 of the optical heterodynedetector 340, for example using a quadrature signal rotation operationthat mathematically can be described as a matrix multiplication:

$\begin{matrix}{\begin{pmatrix}{CC} & {SS} \\{- {SS}} & {CC}\end{pmatrix} \cdot \begin{pmatrix}V_{1} \\V_{2}\end{pmatrix}} & (8)\end{matrix}$

The operation described by expression (8) yields an I-channel outputsignal V_(I)(t) 356 that is proportional to the transmitter I-channelsignal I(t)=[A₀+A(t)], and a Q-channel output signal V_(Q)(t) 357 thatis proportional to the transmitter Q-channel signal Q(t)=B(t), i.e. i.e.V_(I)(t)˜[A₀+A(t)]=I(t) and V_(Q)(t)˜B(t). Once separately extracted,the I-channel signal and Q-channel signals 356, 357 may be separatelyfed into two digital Rx processors 361, 362, which may be for example inthe form, or include, suitable serialilzer—deserializer (SerDes) chips,for further separate I-channel and Q-channel data signal processing asknown in the art.

Accordingly, in one embodiment the output signals 341, 342 from thecoherent EO converter 340 may be fed into an IQ demodulator 350, whichperforms the heterodyne phase recovery operation and recovers thetransmitter-generated I-channel and Q-channel signals. In theillustrated embodiment the IQ demodulator 350 includes a two-channel PLLcircuit 352 and an HF demodulation circuit 354, which may also bereferred to as the IQ rotator 354. One copy of the electrical signals341, 342 is fed into the PLL 352, which operates as a narrow-band filterthat is tuned to the HF ω_(HF) and effectively filters out data-ratemodulation, outputting as its output signals 351 and 353 the twoquadrature HF tones CC(t) and SS(t), preserving their relative phase.Note that in the context of this disclosure, the term ‘PLL” refers toany narrow-band filter that is capable of performing that operation. TheIQ rotator circuit 354 may be configured to perform the signalrotation/matrix multiplication operation described by equation (8). Itdemultiplexes the I-channel and Q-channel signals A(t) and B(t) bydecoupling them from the HF tones, and feeds the extracted transmitterI-channel and Q-channel signals separately in the form of the electricaloutput data signals V_(I)(t) 356 and V_(Q)(t) 357 to the I-channel andQ-channel processors 361, 362.

In one embodiment the IQ demodulator 350 may be embodied using analogelectrical circuitry. Referring to FIG. 4, the signal rotator/multipliercircuit 354 may be implemented for example as a network of analog ordigital signals multipliers 410 and signal adders 420, as would beevident to those skilled in the art. The structure of this network canbe understood by noting that the matrix operation given by expression(8) results in the following two equations defining the I-channel outputsignal V_(I) and the Q-channel output signal V_(Q):

V _(I)(t)=CC(t)* V ₁(t)+SS(t)*V ₂(t)   (9)

V _(Q)(t)=CC(t)*V ₂(t)−SS(t)*V ₁(t)   (10)

The IQ demodulator 350 may also be embodied using digital electronics,or a combination of digital and analog circuits. For example, the PLL352 may be embodied as an analog circuit, while the signal rotator 354may be implemented using digital logic, for example in a microprocessor,an FPGA, or an ASIC.

The heterodyne frequency (UHF, i.e. the frequency difference between thesignal light 301 and the LO light 302, preferably exceeds the linewidthof both the signal and LO light and may be selected for example in therange from about 10 MHz to about 100 MHz, depending on the LO and Txlight linewidths. For example the frequency difference between thesignal and LO light may be about 50 +/−10 MHz, which enables the use ofrelatively low-frequency PLL 352.

The LO source 305 may be for example a frequency-tunable semiconductorlaser that may be similar to that used at the transmitter and whichoptical frequency may be actively tuned to maintain the desiredfrequency offset from the Tx laser when the output frequency of the Txlaser drifts. This can be achieved using a feedback from the PLL 352,which generates signals that are sensitive to HF variations.

Turning to FIG. 5A, there is illustrated a nested Mach-Zehnder modulator(MZM) that may be used at the Tx site to generate a QM optical signal.It is formed of two inner MZMs 121, 122 disposed in two arms of an outerMach-Zehnder interferometer (MZI) 100. Each inner MZM includes a signalelectrode 121 or 122 and a bias electrode 131 or 132. In operation thesignal electrode of one of the inner MZMs receives the I-channelmodulating signal I(t), while the signal electrode of the other of theinner MZMs receives the Q-channel modulating signal Q(t). The outer MZI100 includes a bias electrode 133 that sets the relative optical phasebetween the inner MZM's outputs at π/2 at the output optical combiner.FIG. 5B illustrates an optical transfer characteristic of an MZM, i.e.the dependence of the MZM transmission T on a bias voltage Vb.Conventionally the bias electrodes 131, 132 of the inner MZMs 121, 122set the operating points of the respective MZM to a minimum in its EOtransfer characteristic, which corresponds for example to a bias voltageVb₁ indicated in FIG. 5B. This bias setting results in so calledpush-pull modulation of light in each of the I and Q channels. The DCoffset 222 in the QM signal constellation may be provided, for example,by slightly offsetting the bias voltage applied to one of the innerMZMs, increasing or decreasing it, for example increasing it by a smallvoltage offset 121 from Vb₁ to Vb₂ as illustrated in FIG. 5B. By way ofexample, this bias voltage offset 121 may be in the range of 0.001Vπ to0.1Vπ. In other embodiments the DC offset 222 may be added in theelectrical domain to the amplitude of one of the electrical I-channel orQ-channel modulating signal, and both inner MZMs biased at a minimum ofthe transfer characteristic.

An optical transmitter may be configured to transmit polarizationmultiplexed (PM) QM light wherein two QM optical signals are mixedtogether in orthogonal polarization states, which are typically referredto as the X-polarization and the Y-polarization, providing for adoubling of the number of information channels carried by a singlewavelength. At the receiver site these two PM light signals, which maybe referred to as the X-light and Y-light, have to be separated so thattheir respective I and Q modulation signals may be separately extracted.However, during the propagation through the optical link 21, these twoTx-defined polarizations may become scrambled in a time dependent way,which complicates their separation at the receiver. The separation ofthe transmitter-defined X- and Y-polarization channels at the receivermay be assisted however by adding a distinct dither signal to one of thetwo polarization channels at the transmitter.

Referring now to FIG. 6, there is schematically illustrated an exampleembodiment of an optical PM-QM transmitter 500 in which the Y-light ismodulated with a dither tone at a distinct dither frequency f, orgenerally any distinct dither signal, prior to the polarizationmultiplexing with the X-light. Transmitter 500 may be implemented as aphotonic integrated circuit (PIC), for example in a SOI chip usingplanar silicon waveguides, or another suitable material system. Inputlight 501 from a coherent light source, such as a suitablesingle-frequency semiconductor laser (not shown), is fed into apolarization beam splitter (PBS) 511, which splits it into the X-lightand Y-light of two orthogonal polarizations, optionally rotating thepolarization of one of them to match a preferred polarization mode ofthe PIC waveguides. For example PBS 511 may split input light 101 intoTE mode (X-light) and TM mode (Y-light), and then converting the Y lightfrom the TM to the TE mode. The X and Y lights are then separatelymodulated by two QM optical modulators 510, and then polarizationmultiplexed in orthogonal polarization states using a polarization beamcombiner (PBC) 513 to obtain a PM-QM optical signal 502, which may thenbe transmitted to an optical receiver over a fiber optic link. PBC 511may include a polarization rotator, or TE to TM converter, in one of itinput ports. Each of the QM optical modulators 510 may be embodied, forexample, as a nested MZM illustrated in FIG. 5A. Prior to thepolarization multiplexing, one of the X-light and Y-light may bemodulated in amplitude at the distinct dither frequency f to facilitatethe polarization demultiplexing at the receiver. The dither modulationmay be performed for example using a suitable variable opticalattenuator (VOA) 520. The dither frequency f may be selected to be muchlower than the data rate of the transmitter, for example in the kHz orlow-MHz range, to simplify the generation of the dither signal and itsdetection at the receiver, and to reduce its interference with thetransmitted data signal. The depth of the dither modulation may also besuitably low, for example a few percent, so as not to induce data errorsat the receiver.

Turning now to FIG. 7, there is schematically illustrated an opticalPM-QM receiver 600 that is configured to implement polarizationde-multiplexing of the PM-QM light 502 after its propagation in afiber-optic link or another communication channel with polarization modedispersion. Receiver 600 includes a tunable polarizationsplitter/controller 610 at its input that is configured to tunably splitthe received light into two light signals 611 and 612 in dependence onthe polarization state of the received light. Polarization controllerscapable of tunable polarization splitting that is fully adjustable byvarying two or more polarization control signals are known in the art,and may be for example in the form of a PBS followed by a sequence of2×2 optical couplers interspersed with two or more tunable phaseshifters. A monitoring photodetector (MPD) 620 may be coupled at one ofthe output ports of the tunable polarization splitter 610, and may beconfigured to detect the presence of the dither frequency f in thecorresponding light signal, and to output a dither strength signalindicative of the strength of the dither signal detected by MPD 620. Thedither strength signal may then be provided to a control circuit 625that is configured to vary the polarization control signal or signals soas to minimize or maximize the strength of the detected dither signal,thereby aligning the two light signals 611, 612 to the transmitterpolarization channels X and Y.

By way of example, in one embodiment wherein the Y-light component ofthe PM-QM light 502 was modulated at the transmitter at the ditherfrequency f, the control circuit 625 may be configured to adjust thetunable polarization splitter 610 until the dither frequency f at theoutput of MPD 620 is maximized. In another embodiment wherein theX-light component of the PM-QM light 501 was modulated at thetransmitter at the dither frequency f, tunable polarization splitter 610may be adjusted until the dither frequency f at the output of MPD 620 isminimized. In both cases splitter 610, after the adjustment, splits theinput light into the X-light and Y-light as generated at the transmittersite. These de-multiplexed X- and Y-optical signals 611, 612 may then beseparately fed into two QM receivers 650X and 650Y for demodulation andfurther processing. Each of the QM receivers 650 may be embodied forexample as illustrated in FIG. 3, and may include an optical heterodynedetector 340 followed by an IQ demodulation circuit 630, which is inturn followed by an I-channel signal processor 6401 and a Q-channelsignal processor 640Q. Signal processors 6401, 640Q may each be in theform, or include, a SerDes (serializer—deserializer) chip as known inthe art.

Turning now to FIG. 8, a method 800 for IQ demodulation of QM light inaccordance with an aspect of the present disclosure may include thesteps or operations shown in the flowchart. Step or operation 810 isperformed at a transmitter site, by providing an offset in one of theI-channel and Q-channel signals, for example as described hereinabovewith reference to FIGS. 2B, 3, 5A and 5B. This offset may be in the formof a constant amplitude offset that is added to the amplitude of one ofthe I and Q modulation signals in each QM symbol interval. Steps oroperations 820-840 are performed at a receiver site. At step oroperation 820, optical quadrature heterodyne detection of the QM lightis performed to obtain two electrical signals V₁(t) and V₂(t), each ofwhich comprising the I-channel signal modulated at a heterodynefrequency (HF), the Q-channel signal modulated at the HF ω_(HF), and aheterodyne frequency (HF) tone, for example as may be represented byequations (4) and (5). The two electrical signals V₁(t) and V₂(t) may beviewed as coordinates of a vector [I(t, Q(t)] rotated by an angle equalto a yet unknown heterodyne phaseϕ_(HF)(t)=((ω_(t)−ω_(l))t+(ϕ_(t)−ϕ_(l))).

At step or operation 830, the two electrical signals V₁(t) and V₂(t) arefiltered with a narrow-band phase-sensitive filter to extract therefromtwo quadrature HF tones (6) and (7) while preserving their relativephase. The operation 830 may be referred to as the phase recoveryoperation that recovers the heterodyne phase of the HF modulation for anunknown LO phase. At step or operation 840, the two quadrature HF tonesobtained at 830 are used to decompose the I-channel and Q-channelsignals from the two electrical signals V₁(t) and V₂(t). This operationmay be viewed as a rotation of vector [V₁(t), V₂(t)] by the HF phaserecovery angle ϕ_(HF)(t), which effectively decouples the transmitterI-channel and Q-channel signals from the electrical signals V₁(t) andV₂(t) at the output of the heterodyne QM detector 340.

Advantageously, the approach to coherent detection of optical QM signalsdescribed hereinabove enables to perform direct LO phase recovery usingrelatively simple electrical circuits that may be implemented in analogelectronics or using relatively simple digital electronics, such as fewinterconnected signal multipliers and adders, and do not require ahigh-power, expensive integrated circuit known as a DSP (digital signalprocessor). Thus, the aforedescribed approach enables to perform IQdemodulation at significant saving in cost and system power consumption.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentinvention. Indeed, various other embodiments and modifications to thepresent disclosure, in addition to those described herein, will beapparent to those of ordinary skill in the art from the foregoingdescription and accompanying drawings.

For example, it will be appreciated that different dielectric materialsand semiconductor materials other than silicon, including but notlimited to compound semiconductor materials of groups commonly referredto as A3B5 and A2B4, such as GaAs, InP, and their alloys and compounds,may be used to fabricate the optical circuits example embodiments ofwhich are described hereinabove.

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. An apparatus for coherent demodulation ofquadrature-modulated (QM) light, the QM light comprising two transmittersignals optically combined in quadrature, the apparatus comprising: anoptical quadrature heterodyne receiver configured to receive the QMlight and to obtain therefrom two electrical signals; a phase-sensitivefilter circuit configured to detect a heterodyne frequency (HF) tone ineach of the two electrical signals so as to preserve a relative phasetherebetween; and, a demodulator circuit configured to decompose the twotransmitter signals from the two electrical signals based at least inpart on the HF tones to obtain two decomposed transmitter signals. 2.The apparatus of claim 1 wherein the optical quadrature heterodynereceiver comprises an optical mixer configured to mix the QM light withlocal oscillator (LO) light, wherein the LO light is shifted infrequency from the QM light by a heterodyne frequency ω_(HF).
 3. Theapparatus of claim 2 wherein the optical quadrature heterodyne receiveris configured to provide the two electrical signals comprising each adifferent combination of the two transmitter signals modulated by theheterodyne frequency WHF.
 4. The apparatus of claim 3 wherein theoptical mixer comprises a 90° optical hybrid (OH), the 90° OH comprisingfour output ports.
 5. The apparatus of claim 4 wherein the opticalquadrature heterodyne receiver comprises two differential photodetectorscoupled to the four output ports of the 90° OH for obtaining the twoelectrical signals.
 6. The apparatus of claim 2 wherein the opticalquadrature heterodyne receiver comprises a source of the localoscillator (LO) light.
 7. The apparatus of claim 1 comprising twoelectrical processing circuits coupled to the demodulator circuit forindividual processing of the two transmitter signals.
 8. The apparatusof claim 2 wherein the phase-sensitive filter circuit is configured toextract two quadrature HF tones from the two electrical signals.
 9. Theapparatus of claim 8 wherein the phase-sensitive filter circuitcomprises one or more phase lock loops (PLL) circuits for extracting thetwo quadrature HF tones.
 10. The apparatus of claim 8 wherein the one ormore phase lock loops (PLL) circuits are locked to the heterodynefrequency ω_(HF).
 11. The apparatus of claim 8 wherein the demodulatorcircuit comprises multiplying circuitry configured to multiply the twoelectrical signals by the two quadrature HF tones.
 12. The apparatus ofclaim 11 wherein the multiplying circuitry is configured to multiply oneof the two electrical signals by one of the two quadrature HF tones, tomultiply the other of the two electrical signals by the other of the twoquadrature HF tones, and to sum resulting multiplied signals.
 13. Theapparatus of claim 11 wherein the demodulator circuit comprises foursignal multiplication circuits and two signal summing circuits.
 14. Theapparatus of claim 1 wherein the demodulator circuit comprises analoguecircuitry.
 15. The apparatus of claim 1 wherein the phase-sensitivefilter circuit comprises analogue circuitry.
 16. The apparatus of claim1 wherein the two transmitter signals comprise an I-channel signal and aQ-channel signal, and wherein the demodulator circuit is configured todecompose each of the I-channel and Q-channel signals from the twoelectrical signals using the HF tones.